# What is the chemical composition of Ferrous Ferrocyanide?

To this day, iron-cyanide complexes confuses me. So, I want to settle this once and for all. After scavenging the internet, I gathered the following result:

$$\ce{Ferr} \color{blue}{\ce{ous}}~\ce{Ferr}\color{blue}{\ce{i}}\ce{cyanide -> Fe3^{II}[Fe^{III}(CN)6]2}$$

$$\ce{Ferr} \color{blue}{\ce{ous}}~\ce{Ferr}\color{blue}{\ce{o}}\ce{cyanide ->} \color{reD}{\ce{???}}$$

$$\ce{Ferr} \color{blue}{\ce{ic}}~\ce{Ferr}\color{blue}{\ce{i}}\ce{cyanide (Prussian}~\color{green}{\ce{Green}} \ce{)-> Fe^{III}[Fe^{III}(CN)6]}$$

$$\ce{Ferr} \color{blue}{\ce{ic}}~\ce{Ferr}\color{blue}{\ce{i}}\ce{cyanide (Prussian}~\color{brown}{\ce{Brown}} \ce{)-> Fe^{III}Fe^{III}(CN)6.2H2O}$$

$$\ce{Ferr} \color{blue}{\ce{ic}}~\ce{Ferr}\color{blue}{\ce{o}}\ce{cyanide (Prussian}~\color{blue}{\ce{Blue}} \ce{)-> Fe4^{III}[Fe^{II}(CN)6]3}$$

There are some mentions of "ferrous ferrocyanide" on the internet but I couldn't find its formula.

1. Prussian blue (also known as Berlin blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts (Prussian Blue - Wikipedia)

2. Comment section of this question "What's the chemical formula of "Everitt's salt"?"
3. Evidence in the literature on the synthesis of ferrous ferricyanide is critically discussed. Pyrolysis and pressure effects on Prussian Blue lead to ferrous ferrocyanide together with decomposition by-products, and not to ferrous ferricyanide. The latter compound could be a precursor in the formation of Turnbull's Blue or an excited state of Prussian Blue, but it is not a stable chemical species.

Reguera, E., Fernández-Bertrán, J. & Balmaseda, J. The existence of ferrous ferricyanide. Transition Metal Chemistry 24, 648–654 (1999). https://doi.org/10.1023/A:1006942415737

4. Memoirs and Proceedings of the Manchester Literary & Philosophical Society: (Manchester Memoirs.)., Manchester Literary and Philosophical Society, 1909

Question: What is the chemical formula for "Ferrous Ferrocyanide"?

To be honest, I don't even know the correct formula for Prussian blue. There are so many formula for PB, some of which has potassium included. Moreover, "Turnbull Blue" has similar kind of formula adding more confusion.

It is not common to find the ferrous ferrocyanide because it oxidizes rapidly. As you can see the remaining salts or pigments are pretty stable in air. This one is white. I quote from a very old Analytical Chemistry book by Treadwell, pg. 155, this should address your query. It states:

Potassium Ferrocyanide, $$\ce{K4[Fe(CN)6]}$$, produces in solutions of ferrous salts, with complete exclusion of air, a white precipitate of potassium ferrous ferrocyanide or of ferrous ferrocyanide, depending upon whether one or two molecules of ferrous salt react with one molecule of potassium ferrocyanide:

$$\ce{K4[Fe(CN)6] + FeSO4 <=> K2SO4 + K2Fe[Fe(CN)6]}$$

$$\ce{K4[Fe(CN)6] + 2 FeSO4 <=> 2 K2SO4 + Fe2[Fe(CN)6]}$$

Although both of the above salts are white, a light-blue color is almost always obtained, because the precipitate is immediately oxidized somewhat by the air, forming the ferric salt of hydroferrocyanic acid (Prussian blue):

$$\ce{6 Fe2[Fe(CN)6] + 6 H2O + 3 O2 <=> \underset{\text{Prussian blue}}{\ce{2 Fe4[Fe(CN)6]3}} + 4 Fe(OH)3}$$

• @bittonwood short of time for a major format change. Sep 11, 2021 at 18:47

And then don't forget we have nitroprusside, still used as a short-term, if fairly rough way to reduce peripheral hypertension in a hypertensive crisis (and on the WHO list of essential medications). I'm not sure of its Fe oxidation state, but I'm sure it's one of the above. A remarkable drug, still in use, if even for dramatic (and short term) purposes.